2.1. Chimeric Antibodies
Since the development of the cell fusion technique for the production of monoclonal antibodies (Kohler and Milstein, 1975, Nature (London) 256:495) a vast number of monoclonal antibodies, many of which define heretofore unknown antigens, have been produced by a number of researchers. Unfortunately, most of the monoclonal antibodies made to date are produced in a murine system and, therefore, have limited utility as human therapeutic agents unless modified in some way so that the murine monoclonal antibodies are not "recognized" as foreign epitopes and "neutralized" by the human immune system. A number of researchers, therefore, are attempting to develop human monoclonal antibodies, which are "recognized" less well as foreign epitopes and may overcome the problems associated with the use of monoclonal antibodies in humans. Obviously, the hybridoma technique developed by Kohler and Milstein (supra) which involves sacrificing the immunized mice and using their spleens as a source of lymphocytes for subsequent fusion to immortalize antibody producing cell lines cannot be practiced in humans. Therefore, a number of researchers have directed their attention to recent advances in the field of molecular biology that allow for the introduction of DNA into mammalian cells to obtain expression of immunoglobulin genes (Oi et al., 1983 Proc. Natl. Acad. Sci. USA 80:825; Potter et al., 1984, Proc. Natl. Acad. Sci. USA 81:7161), and have used these techniques to produce chimeric antibodies (Morrison et al., 1984, Proc. Natl. Acad. Sci. USA 81:6581; Sahagan et al. 1986, J. Immunol. 137:1066; Sun et al., 1987, Proc. Natl. Acad. Sci. 84:214).
Chimeric antibodies are immunoglobulin molecules comprising a human and non-human portion. More specifically, the antigen combining region (or variable region) of a chimeric antibody is derived from a non-human source (e.g., murine) and the constant region of the chimeric antibody (which confers biological effector function to the immunoglobulin) is derived from a human source. The chimeric antibody should have the antigen binding specificity of the non-human antibody molecule and the effector function conferred by the human antibody molecule.
In general, the procedures used to produce these chimeric antibodies consist of the following steps (the order of some steps may be interchanged):
(a) identifying and cloning the correct gene segment encoding the antigen binding portion of the antibody molecule; this gene segment (known as the VDJ, variable, diversity and joining regions for heavy chains or VJ, variable, joining regions for light chains (or simply as the V or Variable region) may be in either the cDNA or genomic form;
(b) cloning the gene segments encoding the constant region or desired part thereof;
(c) ligating the variable region with the constant region so that the complete chimeric antibody is encoded in a transcribable and translatable form;
(d) ligating this construct into a vector containing a selectable marker and gene control regions such as promoters, enhancers and poly(A) addition signals;
(e) amplifying this construct in bacteria;
(f) introducing the DNA into eukaryotic cells (transfection) most often mammalian lymphocytes;
(g) selecting for cells expressing the selectable marker;
(h) screening for cells expressing the desired chimeric antibody; and
(i) testing the antibody for appropriate binding specificity and effector functions.
Antibodies of several distinct antigen binding specificities have been manipulated by these protocols to produce chimeric proteins (e.g., anti-TNP: Boulianne et al., 1984, Nature Vol. 312 pg. 643; and anti-tumor antigens: Sahagan et al., 1986, J. Immunol. Vol. 137:1066). Likewise several different effector functions have been achieved by linking new sequences to those encoding the antigen binding region. Some of these include enzymes (Neuberger et al., 1984, Nature 312:604), immunoglobulin constant regions from another species and constant regions of another immunoglobulin chain (Sharon et al., 1984, Nature 309:364; Tan et al., 1985, J. Immunol. Vol. 135:3565-3567).